1. Field of the Invention
The present invention relates generally to a printing apparatus, and more particularly to a printing apparatus having a platen which is pivotally supported.
2. Discussion of the Prior Art
There is known a thermal printer wherein a thermal print head has heat-generating elements adapted to generate heat for fusing local areas of a thermal print ribbon, or alternatively, a print head has recording electrodes adapted to apply an electric current to local areas of an electrically resistive layer of a print ribbon, for causing the energized areas of the resistive layer to produce heat for fusing the corresponding areas of an ink layer of the ribbon. In this type of printing apparatus, a printing operation is effected while a recording medium is pressed by the print head against a platen of the printer, which is generally provided with a flat support surface. Accordingly, the print quality is affected by the contacting condition between the print head and the recording medium, i.e., between the print head and the platen. In view of this fact, it is proposed to provide suitable means for supporting the platen such that the platen is freely pivotable, to permit automatic angular adjustment of the platen surface, following the surface of the print head, when the printhead is moved from its retracted position to its printing position.
Various structures for pivotally supporting the platen are known and used. A typical example of such known support structure uses a support shaft for pivotally supporting a support member on which the platen is mounted. The support shaft is supported by holder members which are attached to a frame structure or housing of the printer.
The above known arrangement uses a relatively large number of component parts, and is therefore difficult to assemble, leading to an increased cost of manufacture of the printer.
Another type of known arrangement for pivotally supporting the platen uses a platen support member which is made of a flexible material. The support member is provided with shaft portions at its opposite ends. On the other hand, left and right side frames have retainer holes for receiving the shaft portions of the flexible support member. When the platen support member is mounted on the side frames, the support member is first flexed or warped in a direction perpendicular to its longitudinal direction, and then the shaft portions are inserted into the respective retainer holes. The support member recovers its original straight shape after the shaft portions are received by the holes.
Although the above arrangement uses a relatively small number of parts and is comparatively easy to assemble, the flexible support member tends to be elastically deformed due to a force exerted by the print head, and the print quality is deteriorated, during a sustained period of use of the printer.